The donor specificity and kinetics of the hydrolysis reaction of γ-glutamyltransferase

Abstract

The donor specificity of the hydrolytic reaction of γ-glutamyltransferase ((5-glutamyl)-peptide:amino-acid 5-glutamyltransferase, EC 2.3.2.2) has been studied by the use of specifically synthesised γ-glutamyl substrates. It was found that a wide variety of γ-glutamylated adducts were hydrolysed by the enzyme. The structure of the adduct was relatively unimportant for donor specificity and the enzyme appears to ‘recognise’ the γ-glutamyl portion of the donor molecule. In particular the α-amino group and the free proton of the γ-peptide bond appear to be essential for donor activity. The V max of hydrolysis increased proportionally to the electron-withdrawing capacity of the adduct moiety. The rate of formation of γ-glutamyl-enzyme intermediate was therefore dependent upon the structure of the adduct of the γ-glutamyl donor. The results suggest that the enzyme shows little specificity beyond that for γ-glutamyl amides and there is therefore no reason to postulate the presence of a specific glutathione-binding site.